The invention relates to induction-heated guide-tubes for pouring of liquid metal. In particular, the invention relates to pouring of superalloys during electroslag refining using a copper guide tube that concentrates flux in the liquid metal stream.
Electroslag refining is a process used to melt and refine a wide range of alloys, including but not limited to superalloys, for removing various impurities therefrom. U.S. Pat. No. 5,160,532, issued to Benz et al., discloses an electroslag refining apparatus that is assigned to the Assignee of the present invention, General Electric. Other ESR structures are set forth in several US patents issued to the Assignee of the present invention, General Electric, including U.S. Pat. Nos. 5,310,165; 5,325,906; 5,332,197; 5,348,666; 5,366,206; 5,472,097; 5,649,992; 5,649,993, 5,683,653, 5,769,151; 5,809,057; and 5,810,066, the contents of each are incorporated herein.
In general, an electroslag refining apparatus comprises an ingot being connected to a power supply, for example one of an alternating or direct current power supply. The ingots comprise unrefined alloys that may include various defects or impurities, which are desired to be removed during the refining process to enhance its metallurgical properties, including, but are not limited to, grain size and microstructure. The ingot forms a consumable electrode that is suitably suspended in a water-cooled crucible, which contains a slag corresponding with the alloy being refined. The slag is heated by passing an electrical current from the electrode through the slag into the crucible. The slag is maintained at a suitable high temperature for melting the lower end of the consumable electrode into an ingot melt. As the consumable electrode melts, a refining action takes place with oxide inclusions in the ingot melt being exposed to the liquid slag and dissolved therein. Liquid refined melt of the ingot melt fall through the slag by gravity, which may be augmented or diminished by additional electromagnetic forces. The liquid refined melt is collected in a liquid melt pool at the bottom of the crucible. The slag, therefore, effectively removes various impurities from the melt to effect the refining thereof. The refined melt may be extracted from the crucible by an induction-heated, segmented, water-cooled copper guide tube. The refined melt extracted from the crucible thus provides a liquid metal source for various solidification processes including, but not limited to, powder atomization, spray deposition, spray forming investment casting, melt-spinning, nucleated-casting, strip casting, and slab casting.
In above-described electroslag refining apparatus, the crucible can be formed of copper, and is typically water-cooled to form a solid slag and/or metal skull on its surface. The solid slag or metal skull bounds the liquid slag and prevents damage to the crucible itself The bottom of the crucible typically includes a water-cooled, cold hearth, which can be formed of copper, against which a solid skull of the refined melt forms for maintaining the purity of the collected melt at the bottom of the crucible. A discharge guide tube assembly below the hearth can also be formed of copper. The discharge guide tube assembly is often segmented and water-cooled and allows the formation of a solid skull of the refined melt for maintaining the purity of the melt as it is extracted from the crucible. The skulls can prevent contamination of the ingot melt from contact with the parent material of the crucible.
The electroslag refining apparatus also may include a plurality of water-cooled induction heating electrical conduits that surround a discharge guide tube. The conduits inductively heat the melt and the discharge guide tube can control the discharge flow rate through the discharge guide tube. Accordingly, the thickness of the skull formed around the discharge orifice may be control led and suitably matched with melting rates of the consumable electrode for obtaining a substantially steady state production through the discharge guide tube.
The discharge guide tube and cold hearth of some electroslag refining apparatuses are generally structurally complex, and are generally comprise a plurality of fingers or segments, which are surrounded by the induction heating electrical conduits. These induction heating electrical conduits are often single piece units that are typically provided with a set configuration to conform with the configuration of the discharge guide tube. The configuration is provided to define a gap between the induction heating electrical conduits and the discharge guide tube. This configurations is suitable for heating the melt in and about the discharge guide tube in electroslag refining applications. However, if one or both of the induction heating electrical conduits and discharge guide tubes are moved with respect to one another, the gap therebetween changes due to the single-piece configuration of the induction heating electrical conduits. Therefore, the heating of the melt in and about the discharge guide tube in electroslag refining applications may be influenced, often detrimentally.
Further, each of the discharge guide tube's segments may be manufactured with internal cooling passages, which adds to the complexity of the assembly and cost of manufacture. The discharge guide tube is typically integrally joined to the cold hearth with the induction heating coils surrounding its outer surface. Typically, these discharge guide tubes require numerous and complex manufacturing and machining to be formed, including specialty milling. Thus, the discharge guide tube and cold hearth are expensive to manufacture.
The above-described electrical conduits generate an electromagnetic field, and an associated electromagnetic flux within the discharge guide tube, thus heating any material flowing therethrough. The intensity of the generated electromagnetic field and electromagnetic flux is typically related to the heating capability of the guide tube apparatus. As the electromagnetic field and electromagnetic flux intensities increase, the heating capability within the discharge guide tube increases. A high field intensity and electromagnetic flux, and resultant high heating capability in a guide tube apparatus, is often desirable for creating an initial stream of metal, melting any undesired solid metal within the electroslag refining apparatus, and super-heating the stream flowing through the discharge guide tube.
The electromagnetic flux intensity in the guide tube apparatus can be enhanced by providing at least one of a high applied voltage in the electrical conduits and an increased number of induction heating elements disposed about the guide tube apparatus. The configuration and structure of the guide tube apparatus can limit a number of induction heating elements. Further, the current amount is limited by the configuration and structure of the induction heating elements and an availability of electrical energy. Thus, the guide tube apparatus may be limited in its the capability to enhance the electromagnetic field and resulting electromagnetic flux intensity in the discharge guide tube.
Accordingly, a need exists to enhance the electromagnetic field, and resulting electromagnetic flux in copper guide tube apparatuses. In particular, a need exists to enhance the electromagnetic flux concentration without significant redesign of the guide tube apparatus.